1. Field of the Invention
The present invention relates to a thixocasting process and particularly, to an improvement in a thixocasting process including the steps of: subjecting to a heating treatment, an alloy material having a differential calorimetric curve in which a first angled endothermic section generated by the melting of a eutectic crystal and a second angled endothermic section generated by the melting of a component having a melting point higher than a eutectic point exist, thereby producing a semi-molten alloy material having a solid phase (which means, throughout the present specification, a substantially solid phase) and a liquid phase coexisting therein, and pressing the semi-molten alloy material to conduct the charging of the semi-molten alloy material into a cavity in a casting mold and the subsequent solidification of the semi-molten alloy material under pressure.
2. Description of the Prior Art
In the prior art, the pressure applied to the semi-molten alloy material is set such that it is rapidly and rectilinearly raised to a predetermined value after charging of the material into the cavity in the casting mold. The reason why the pressure is applied in such manner is that the liquid phase is supplied to portions of the material around the solid phase to prevent the generation of shrinkage cavities.
In this case, a portion around the outer periphery of the solid phase in the semi-molten alloy material filled in the cavity in the casting mold is in a gelled state, and such gelled layer obstructs the flow property of the liquid phase. In order to overcome such obstruction to permit the liquid phase to flow, the pressure is set at a very high value, e.g., in a range of 850 to 2,000 kg f/cm.sup.2 in the terms of a plunger pressure.
However, to set the plunger pressure at such a high value as described above, large-sized equipment is required, resulting in a problem that an increase in equipment cost and in turn, an increase in production cost of the cast product, is brought about.
As a high-toughness alloy material, e.g., as a high-toughness aluminum alloy material AA specification 6000-series alloys are known.
However, when the known 6000-series alloy is used in the thixocasting process, the following problem is encountered: defects such as voids of micron order are liable to be generated at a grain boundary in a cast product, and the fatigue strength of the cast product is low. Such defects are generated due to the fact that supplying of the liquid phase to portions around the solid phase is not conducted in response to the solidification and shrinkage of the solid phase, because the liquid phase produced due to the melting of a eutectic crystal hardly exists in the 6000-series alloy material in a semi-molten state.
Further, for example, an AA specification 238 alloy material containing copper (Cu) with a content of 9.5% by weight.ltoreq.Cu.ltoreq.10.5% by weight and silicon (Si) with a content of 3.5% by weight.ltoreq.Si.ltoreq.4.5% by weight is known as a thixocasting Al--Cu--Si based alloy material.
However, when the known 238 alloy material is used in the thixocasting process, the following problem is encountered: voids of micron order are liable to be generated at a boundary between granular solid phases in an aluminum cast product. This is for a reason which will be described below. The known 238 alloy material has because of a large content of Si, a thermal characteristic that in a first angled endothermic section in a differential calorimetric curve, the inclination of rising a line segment located between a rise-start point and a peak is gentle, resulting in an increased viscosity of a final solidified portion of the liquid phase and hence, the liquid phase is not sufficiently supplied to portions around the solid phase in response to the solidification and shrinkage of the solid phase.